Not much bigger than Earth's moon, Mercury once was written off as a barren world too boring to bother with. Now, many researchers argue that Mercury holds the key to understanding the conditions from which the diverse range of inner planets formed some 4.6 billion years ago. And Mercury is expected to shed light on other star-hugging terrestrial planets that are expected to pop into view when a new generation of space-based telescopes is sent into orbit over the next 10 to 15 years.

"A mission to Mercury is really a mission to the innermost part of the nebula out of which planets formed," says Sean Solomon, director of the terrestrial magnetism department at the Carnegie Institution of Washington and the mission's lead investigator.

Until now, the US space program has largely ignored Mercury, notes Robert Strom, a University of Arizona planetary scientist who with colleague Ann Sprague published a book about the planet last year. "Mercury looked too much like the moon to a lot of people, and sending an orbiter there was extremely difficult," he says.

How difficult? Take heat, for instance. Mercury traces an elliptical orbit that ranges from 28.5 million miles to 43.3 million miles from the sun, compared with Earth's average distance of about 93 million miles. The planet's "day" lasts for two Mercury years - or 176 Earth days. To a hypothetical observer standing on the surface, that works out to 88 Earth days basking under a blazing sun roughly 11 times as hot as it appears at Earth. The temperature difference between Mercury's day side and night side spans 1,100 degrees F. With a tenuous atmosphere - dubbed an exosphere - that fails to trap heat, the surface radiates virtually all its heat back into space.

Thus, engineers need not only to devise a suitable shield to protect a spacecraft from the sun, but also to design instruments that can survive stresses imposed by staring at a surface that cycles from torrid to frigid and back on each orbit.

In addition, the energy demands to reach and circle Mercury are enormous. It's possible - and fastest - to reach the planet by making a beeline for it, as Mariner 10 did the last time NASA flew by 30 years ago. But a craft would gain speed relentlessly because of the sun's gravitational pull, requiring a braking rocket as big and expensive as the one that launched the spacecraft from Earth in the first place, says mission manager Robert Farquhar of the Johns Hopkins University Applied Physics Laboratory in Laurel, Md.

These days, however, new lightweight materials, innovative instrument designs, and the creative use of gravity en route have dramatically cut the cost of sending a long-duration orbiter there and have given it a high probability of surviving in Mercury's demanding environment.

The trade-off: That three-month beeline to Mercury stretches into a 6-1/2 year trip when using the gravity of Earth, Venus, and Mercury itself to bleed enough speed from Messenger to allow Mercury to capture it. Messenger is expected to begin orbiting the planet in 2011.

Messenger carries a suite of instruments covering virtually as wide a range of scientific interests as those carried on behemoths such as the $3 billion Cassini spacecraft now orbiting Saturn. And while its sensors may not be as capable as those on Cassini - for size and weight reasons - it's tackling Cassini-size questions for one-seventh the cost. "We're getting a Mercedes for the price of a Volkswagen," says Dr. Strom, a Mariner 10 veteran who is part of Messenger's core science team.

Yet if changes in technology and a deeper understanding of orbital mechanics now make a visit to Mercury possible, Mariner 10 results and observations over the past 15 years have strengthened the scientific case for returning there.

"Mercury is an enormous gap in our knowledge of the solar system," Strom notes. "It's less well-known than any other planet."

To start with, more than half the planet is missing from earthlings' maps. Mariner 10 flew by Mercury three times. But it returned images of only 45 percent of the planet's surface. Earth-based radar slowly has started to fill some of the gaps. However, researchers say they need Messenger's detailed global images to more easily read the geological history written in its craters, faults, and other surface features.

Beyond maps, "Mercury is going to help us decide on the origin of the inner planets," Strom says. The key lies in testing ideas on how Mercury ended up as the "iron man" of the inner planets. Mercury's iron core constitutes 60 percent of the planet's mass, compared with about 30 percent for Earth's iron core.

Some researchers hold that Mercury formed where it is today. If so, Mercury's hefty center could imply that the nebula that gave rise to the planets was richer in heavy elements like iron close to the sun. Silicates and other, lighter elements progressively dominated at greater distances.

Other researchers hold that the inner nebula was more uniformly mixed, and that initially Mercury had a thicker mantle of silicates, similar to those of the other inner planets. But, they say, Mercury lost much of its rocky covering either because energetic stellar winds early in the sun's evolution blasted the outer layers away, or the material got knocked away when a relatively large planet wannabe collided with Mercury during the solar system's early epochs.

Strom, who favors the collision idea, adds that each theory predicts a somewhat different chemical recipe for the planet's crust and mantle - recipes that Messenger's spectrometers should uncover.

Mercury's global magnetic field represents another mystery. If Mercury's iron core acts anything like Earth's, the field would be generated by the circulation of molten material in the core. Yet by all rights, the planet is too small to have a hot core, says Maria Zuber, a planetary scientist at the Massachusetts Institute of Technology and another Messenger investigator. "The rule of thumb is that big planets heat and cool slowly, and small planets heat and cool quickly," she explains.

If Mercury's outer core is molten, Dr. Zuber continues, the sun's gravity would induce tides in the fluid. These tides would twist the crust at the equator, slowly shifting surface features a few hundred yards as the tides ebb and flow. She and her colleagues have designed exquisitely sensitive instruments for detecting these tiny changes from orbit.

Among its other research objectives, Messenger also will be investigating what Carnegie's Dr. Solomon calls "one of the most bizarre questions: Might the planet have ice at the poles?"

In 1999 astronomers reported that they had bounced radar signals off of Mercury and the return signals strongly suggested the presence of water ice in polar craters. Mercury twirls on an axis that is virtually vertical relative to the plane of its orbit around the sun. Thus the deepest craters at the poles have been in shadows since collisions first carved them into the planet's surface billions of years ago. Temperatures in these craters are thought to hover at minus 300 degrees F. - "cold enough to freeze out volatiles and keep them in a solid form for the lifetime of the planet," Solomon says. Comets would be likely suspects for depositing ice on the planet.

Ice is not the only explanation for what the radio astronomers detected, Solomon acknowledges. These frosty conditions could induce unusual properties in rocks at the surface, leading them to yield a radar return similar to ice. Or the echoes could be coming from frozen sulfur, which also might look to radar like water ice.

Whatever the answer, "this mission is going to give us a treasure trove" of information about Mercury and about how planets form, says MIT's Zuber, who also has been involved in missions to some of the more charismatic planets. "But it's also going to teach us a lot about how to operate spacecraft and instruments in a challenging space environment."

Mercurial realm

• Mercury is roughly the same size as Earth's moon.

• It circles the sun at about 107,000 miles per hour - faster than any planet in the solar system.

• Mercury rotates only three times during two of its years.

• One day on Mercury is 4,226 hours long - 176 Earth days - plenty of time to get extremely hot.

• From night to day, the temperature at Mercury's surface rises some several hundred degrees F. - from as low as minus 320 degrees at night to as high as 845 degrees during its protracted day, enough to melt tin and lead.

• Messenger's circuitous 4.9 billion mile journey to Mercury will take six years.